(1) Field of the Invention
This application is directed to improvements in rolling mill housings used in rolling operations in the flat rolled metal industry. In particular, the present invention is directed toward a multi-roll cluster type of rolling mill.
(2) Description of Related Art
Cluster mills are popular in the rolling mill industry when an ultra-high strength material is rolled, a high gauge reduction is taken, a thin exit gauge is rolled, or any combination of the three. A cluster mill provides many advantages to the operation of a rolling mill and includes the following: small diameter work rolls, high housing stiffness, and a simplified gauge control. In many previous applications, the cluster mill housing has been built based on a mono block design, such as seen and described in U.S. Pat. No. 5,421,184, U.S. Pat. No. 2,187,250 in FIG. 8, and U.S. Pat. No. 2,776,586 in FIG. 8.
In particular, the gauge (or thickness) control is excellent due to the high mill stiffness where entry gauge variances tend to be smoothed out. The higher mill modulus generates smaller mill stretch, which makes stable gauge control. The higher mill stiffness does this because entry gauge spikes are met with a sharp increase in rolling force, and entry gauge drops see a deeper fall off in rolling force, especially when compared to other types of rolling mills and gauge control schemes.
The cluster mill method of gauge control is very simple and is provided by rotating bearing shafts supported within eccentric saddles, which adjust the positions of the backup rolls, which in turn adjust the work roll gap. The developed rolling force is transferred to the mono block housing through the rolls at various angles which add to the mill stiffness. The force needed to rotate the bearing shafts, i.e. to control the exit thickness or exit gauge of the metal strip, is a fraction of the actual rolling force, approximately 3-5%, which simplifies the design of the controlling equipment. The resolution of gauge control becomes much finer thanks to these mechanical advantages. The rotation of the bearing shafts is done mechanically, often by a rack and pinion arrangement, and is driven by a hydraulic system for a fast response, or alternately, is driven by an electro mechanical motor system which is usually slower. The highly leveraged movement also means that the movement of any activation means, such as a rack and pinion, uses equipment based on common commercial machined tolerances, and larger movements of the control system will cause a very fine adjustment in the roll gap.
Though a Cluster mill has historically been attractive for many rolling applications, there still remains a need for improved flexibility in the rolling operation, specific to the needs of an operating plant. In particular, the cluster mill rolling operation is not conveniently designed to operate with different work roll diameters, such as may be used with temper rolling and cold mill rolling. Temper rolling is more optimized for speed and uses larger diameter rolls with a lower rolling force. Cold rolling uses smaller diameter rolls for larger reduction and is optimized for higher rolling forces at reduced speeds. It is advantageous for some plant operations to be able to operate a cold mill, anneal the material, and then operate a temper mill to provide flatness and final material properties. For a low production operation, it is undesirable to purchase redundant cold mills that will not be used all the time. Other advantages of multiple diameter rolling mill operation are readily apparent to a plant operator.
One disadvantage to using a Cluster mill is the rolling force being applied during a strip break. In many cases, a strip break results in many pieces of metal strip remaining within the mono block, and pieces of the metal are likely to wrap around various rolls in the cluster roll arrangement. This is a common, though infrequent, event during the rolling operation. It is helpful to be able to open up a mill, i.e. create a large opening gap between the work rolls, fairly easily and reasonably quickly to clear the unwanted metal out of the mill housing area. This opening can be created off line, when the mill is not in a rolling mode.
During a strip break or loss of tension, it is not necessary in all cases to provide an actual work roll gap opening. In fact, a roll gap opening is disliked by some operators due to the problems associated with keeping small diameter work rolls inside the mill. The rolls are likely to spill out of the mill in the event of a strip break (i.e. ‘mill wreck’) due to the uncontrolled high forces.
In addition, the mono-block Cluster mill has a limited range of work roll diameters that will operate within the design of the mono block. The rotating bearing shafts often allow a very narrow operating range, on the order of 0.10″ at the work roll gap.
Another disadvantage of the Cluster mill is the reduced ability to be flexible for a varied rolling operation. It is highly desirable in some commercial settings to have a single rolling mill capable of cold rolling with a heavy reduction and temper rolling with a light reduction. A temper rolling configuration preferably utilizes a larger work roll size. Larger work rolls allow for a longer work roll life, a faster rolling operation, favorable strip shape, and better rolling feasibility. In contrast, the mono block Cluster mill is unattractive for a mill that is capable of both temper and cold rolling operations.
The mono block is not designed for a convenient and accurate tilting arrangement when there is a significant side to side gauge variance in the metal strip, that is, a wedge shaped strip. Depending upon the upstream hot rolling operation, a metal strip will often have a moderate thickening in the middle of 1 to 3% of the nominal gauge normally, even 5% for some cases. After hot rolling, the strip is sometimes slit into two halves (or more) for further downstream processing which includes rolling on a Cluster mill. This presents a wedge shaped strip to the Cluster mill with an unpredictable thickness across the width. Since the cluster mill design does not include a rolling force measurement, it is difficult to make an accurate side to side rolling gap correction. The rotation of the crown eccentric rings used for profile control often do not provide enough tilting capability, when considering that the operating range may be reduced due to a particular work roll diameter pair in the mill. Consequently, rolling a wedge shaped strip will have problems which include strip breakage, creating camber, creating centerbuckle, creating uneven edge wave, and other unusual strip flatness problems. Improved flexibility is highly desirable.
It is desirable to maintain the amount of mill stiffness by avoiding issues with precision maintenance of high pressures in hydraulic cylinders during the rolling operation.
U.S. Pat. No. 5,142,896 describes a pre-stressed cluster rolling mill using a dual action upper cylinder to create the prestress between the upper and lower mill housings. Though there are certain advantages in operation, the double acting cylinder requires a highly responsive hydraulic control system to overcome the softer mill modulus due to the oil column involved in creating the rolling force. The mill modulus is reduced due to the movement of the top housing and the mechanical advantage is lost due to not using the eccentricity of bearings on rolls B and C and the rack-pinion mechanism for gauge control. This type of rolling mill lacks simplicity for certain markets where low maintenance and minimalistic control that yields high gauge accuracy are greatly preferred. In particular, low production markets desire simplicity in operation and maintenance.
U.S. Pat. No. 6,260,397 considers the need to provide operational improvements that are not available with a mono block. The design does not take advantage of the mono block stiffness, but conceptually splits the mono-block, turning it into a pair of chocks that hold the cluster rolls. The design does not use the simplified gauge control available with a mono block, and is not a prestress design. The design has a relatively low mill stiffness and uses a typical rolling mill gauge control system, such as is seen in a four high or six high mill.
U.S. Pat. No. 5,596,899 by Sendzimir, et al, mentions a prestress mill in the prior art discussion of FIG. 2 and then discusses an improved design in FIG. 4. There are improvements in convenience with respect to mill opening, however the prestress is maintained by a hydraulic cylinder with the subsequent loss of mill housing stiffness. This can be compensated by a suitable highly responsive hydraulic design, however, in some cases it is a less desirable method of providing the high mill modulus. It is preferable to provide simplicity in design implementation, and avoid control interaction to improve overall reliability by eliminating complicated control schemes which operate simultaneously.
U.S. Pat. No. 5,996,388 describes hydraulically preloaded rolling stands utilizing hydraulic prestress rods and hydraulic opening rods. Though a pre-stressed mill is shown, as a practical matter, the design is overly complicated and expensive, and does not take an optimal approach to solving operational issues. The design includes numerous large diameter machined shafts which impact and increase the mill housing design and size. The design is unsuitable for an operation which seeks a low cost and simplified approach to rolling to close tolerances.
There is a need to provide an improved design with a high rolling mill stiffness, a simplified gauge control system, a large work roll gap opening for threading, a method to reduce the work roll force during a strip break, satisfactory side to side tilting during rolling, and is able to use the work rolls over a much wider diameter range. Such a mill is capable of operating satisfactorily as a commercial temper mill and a commercial cold mill in a highly flexible, low cost, and low production environment.